ABSOLUTENESS FOR UNIVERSALLY BAIRE SETS AND THE UNCOUNTABLE II ILIJAS FARAH, RICHARD KETCHERSID, PAUL LARSON, AND MENACHEM MAGIDOR Abstract. Using § and large cardinals we extend results of Magidor{Malitz and Farah{Larson to obtain models correct for the existence of uncountable homogeneous sets for ¯nite-dimensional partitions and universally Baire sets. Furthermore, we show that the constructions in this paper and its predecessor can be modi¯ed to produce a family of 2!1 -many such models so that no two have a stationary, costationary subset of !1 in common. Finally, we extend a result of Steel to show that trees on reals of height !1 which are coded by uni- versally Baire sets have either an uncountable path or an absolute impediment preventing one. In [3] it was shown (using large cardinals) that if a model of a theory T satisfying a certain second-order property P can be forced to exist, then a model of T satisfying P exists already. The properties P considered in [3] included the following. (1) Containing any speci¯ed set of @1-many reals. (2) Correctness about NS!1 . (3) Correctness about any given universally Baire set of reals (with a predicate for this set added to the language). In this paper we add the following properties, all proved under the assumption of Jensen's § principle. (4) Correctness about Magidor{Malitz quanti¯ers (and even about the exis- <! <! tence of uncountable homogeneous sets for subsets of [!1] and any [·] ). (5) Correctness about the countable chain condition for partial orders. (6) Correctness about uncountable chains through (some) trees of height and cardinality !1. (7) Containing a function on !1 dominating any such given function on a club. These results are obtained using two main tools (both due to Woodin): (a) iterable models (also called Pmax-preconditions), introduced in [21], 2 (b) stationary-tower forcing ([11]), or more speci¯cally, Woodin's proof of §1- absoluteness ([20]) . While (b) requires higher large cardinal strength than (a), it allows one to assure (1). Aside from (1) and (7), we can obtain all of these properties simultaneously using the method (a) (with \some" being \all" for (5) and (6)). Aside from (1) and (4) we can prove all of these properties simultaneously using the method (b). Property (4) subsumes the next two properties in the list, but we do not see how to obtain it Date: December 21, 2005. The ¯rst author is partially supported by NSERC. The third author was supported in part by NSF grant DMS-0401603, and part of his research was conducted at the Institute for Mathematical Sciences at the National University of Singapore, under the partial support of a summer research grant from Miami University. 1 2 ILIJAS FARAH, RICHARD KETCHERSID, PAUL LARSON, AND MENACHEM MAGIDOR by stationary tower constructions. As a matter of fact, simultaneously obtaining 2 (1) and (4), and even (1) and (6), would imply §2-absoluteness conditioned on § (see Theorem 3.7). That § implies that one can iterate a Pmax pre-condition to be correct about the countable chain condition for partial orders on !1 is due to Larson and Yorioka [13]. In the fourth section we show abstractly that these arguments can be modi¯ed to build a family of 2!1 many models, no two having a stationary-costationary subset of !1 in common. In the ¯nal section we prove a generalized version of a theorem of Steel which can be used to show that the existence of a model of a given sentence which is correct about a given universally Baire set is absolute, given a proper class of Woodin cardinals. 1. Magidor{Malitz logic The language L(Q<!) is formed by adding to the language of set theory quan- ti¯ers Qn for each n in !. In this paper we restrict our attention to the so-called !1-interpretation of this language. That is, a formula of the form n Q x1; x2; : : : ; xnÁ(x1; : : : ; xn) is interpreted as saying that there is an uncountable subset of !1 such that every n-tuple from this set satis¯es Á. The expressive power of this language is not diminished by requiring Á to be symmetric, i.e., invariant under permuting its n free variables x1; : : : ; xn. Recall that [Z] is the set of all n-element subsets of <! n Z and [Z] is the set of all ¯nite subsets of Z. For K ⊆ [!1] an X ⊆ !1 is K-homogeneous if and only if [X]n ⊆ K. Since an interpretation of a symmetric n formula is a subset of [!1] , correctness for Magidor{Malitz logic is equivalent to correctness for the existence of uncountable homogeneous sets (note that the existence of countable homogeneous sets of any given order type is absolute between transitive models.) We therefore say that a model M is correct for Magidor{Malitz M logic (or for Ramsey quanti¯ers) if !1 is uncountable and, for every n 2 ! and n M every K ½ ([!1] ) de¯nable in M from parameters in M (note that we do not assume here that M satis¯es ZFC), K has an uncountable homogeneous set if and only if one exists in M. The following theorem was proved in [14]. Theorem 1.1 (§). If T is a theory in the language L(Q<!) and it is consistent with ZFC that T has a model which is correct for Magidor{Malitz logic, then T has such a model. ¤ Requiring T from Theorem 1.1 to contain a large enough fragment of ZFC is not a loss of generality. Here (and throughout this paper) \large enough fragment of ZFC" means large enough to make ultrapower embeddings for generic ultra¯lters ± on !1 elementary; the theory ZFC from [12] su±ces. In this case Theorem 1.1 can be equivalently reformulated as follows: Theorem 1.2 (§). If a theory T extends a large enough fragment of ZFC and M it is consistent, then § implies the existence of a model M for T such that !1 is uncountable and M is correct about the existence of homogeneous sets for partitions M n of [!1 ] for all n 2 N. ¤ The model M guaranteed by this result is not necessarily well-founded; asserting well-foundedness of M requires some large cardinal strength (this follows from [3, Proposition 7.10]). A model M of a large enough fragment of ZFC is correct for ABSOLUTENESS FOR UNIVERSALLY BAIRE SETS AND THE UNCOUNTABLE II 3 partitions of ¯nite sets if it is correct about the existence of uncountable homoge- <! neous sets for partitions K ⊆ [!1] in M. Our ¯rst result is a strengthening of Theorem 1.2 (see Proposition 2.8). Theorem 1.3 (§). If a theory T extends a large enough fragment of ZFC and it M is consistent, then § implies the existence of an model M for T such that !1 is uncountable and M is correct about the existence of homogeneous sets for partitions M n of [!1 ] that belong to M, for each n 2 !. If T is !-consistent, then then § implies M the existence of an model M for T such that !1 is uncountable and M is correct M <! about the existence of homogeneous sets for partitions of [!1 ] that belong to M, The proof is given in Theorem 1.12 at the end of this section. We show that in the presence of large cardinals, correctness for partitions of ¯nite sets can be combined with correctness for any given universally Baire set of reals, with respect to the logic of forceability. Analogously to [3, x5], given a set of reals A let L(A) be the language of set theory with an additional unary predicate for A. We say that a model M is correct for <! ! M A and partitions of [!1] (in short, L(Q ;A)-correct) if !1 = !1, M interprets the additional unary symbol as A \ M and about the existence of uncountable <! 1 homogeneous sets for every partition K 2 M of [!1] . Since correctness for ¦1- M M sets already implies well-foundedness of !1 , assuming !1 = !1 is not a loss of generality in our context. The notation L(Q·!;A)-correct is perhaps misleading, but L(Q<!) is an established notation for Magidor{Malitz logic. The reader may wish to compare the following theorem with the results in [1]. Theorem 1.4. Suppose that there exist proper class many Woodin cardinals, let A be a universally Baire set of reals, and let T be a set of sentences in L(A). Suppose that there exists an L(Q!;A)-correct model of T in some set forcing extension. Then there exists an L(Q!;A)-correct model of T in every set forcing extension satisfying §. Proof. Immediate from Lemma 1.5, Lemma 1.6, and Theorem 1.7 below. ¤ <! n The logic L!1!(Q ) allows countable disjunctions in addition to quanti¯ers Q (n 2 N). It is well-known that an analogue of Theorem 1.1 for this logic can be proved using the methods of [14]; for a proof see e.g., [4]. By standard methods (see e.g., [2] for the case of L!1!(Q)), the case of Theorem 1.4 when A is a Borel set follows. This cannot be extended even to analytic sets unless large cardinals are assumed ([3, Proposition 7.8]). An alternative way for proving these results using iterated generic ultrapowers is outlined in our Theorem 1.3 and Theorem 1.12. Note that this semantical result does not recover the full strength of Keisler or Magidor{ Malitz theorems.